You can get SD cards/USB sticks loaded with ROMs from Ali Express if you want to pay money, there are torrents around that are completely free, if your a techie the there is probably someone in your workplace you can get copies from, etc. Nobody really cares as long as you are not making a profit. China doesn't care period. Nobody is going to start a two ocean war over fifty year old video games.
Capcom over the years has had a ROM set you could buy on and off. Not sure if they still do. Think it was side by side with an arcade controller. I think there is one of their current console releases where there are a bunch of games and it uses MAME as the core to run their games. They are the only one I can think of that has sold ROMs to be used in MAME both loose and bundled. There was a service about 20 years ago claiming they had the ability to sell them but it seemed kind of shady. Also it is not very clear cut on many of them anymore who actually owns the game. Most of it has very clear lineage but not all of it.
